The further procedure is quite analogous to the case of a flat spacetime (see, e.g., [1]}, [2]}, [3]}, [4]}). We shall follow [1]}. Substituting the expansion
\(\Big (1-\frac{m^2}{\omega ^2 g^2}\Big )^{d/2-k}=\sum _{n=0}^{\infty }\frac{(-1)^n\Gamma (d/2-k+1)}{n!\Gamma (d/2-k-n+1)}\Big (\frac{m^2}{\omega ^2 g^2}\Big )^n\) 
